An electrical storage battery comprises one electrochemical cell or a plurality of electrochemical cells of the same type, the latter typically being connected in series to provide a higher voltage or in parallel to provide a higher charge capacity than provided by a single cell. An electrochemical cell comprises an electrolyte interposed between and in contact with an anode and a cathode. For a storage battery, the anode comprises an active material that is readily oxidized, and the cathode comprises an active material that is readily reduced. During battery discharge, the anode active material is oxidized and the cathode active material is reduced, so that electrons flow from the anode through an external load to the cathode, and ions flow through the electrolyte between the electrodes.
Many electrochemical cells used for electrical storage applications also include a separator between the anode and the cathode is required to prevent reactants and reaction products present at one electrode from reacting and/or interfering with reactions at the other electrode. To be effective, a battery separator must be electronically insulating, and remain so during the life of the battery, to avoid battery self-discharge via internal shorting between the electrodes. In addition, a battery separator must be both an effective electrolyte transport barrier and a sufficiently good ionic conductor to avoid excessive separator resistance that substantially lowers the discharge voltage.
Electrical storage batteries are classified as either “primary” or “secondary” batteries. Primary batteries involve at least one irreversible electrode reaction and cannot be recharged with useful charge efficiency by applying a reverse voltage. Secondary batteries involve relatively reversible electrode reactions and can be recharged with acceptable loss of charge capacity over numerous charge-discharge cycles. Separator requirements for secondary batteries tend to be more demanding since the separator must survive repeated charge-discharge cycles.
For secondary batteries comprising a highly oxidative cathode, a highly reducing anode, and an alkaline electrolyte, separator requirements are particularly stringent. The separator must be chemically stable in strongly alkaline solution, resist oxidation in contact with the highly oxidizing cathode, and resist reduction in contact with the highly reducing anode. Since ions, especially metal oxide ions, from the cathode can be somewhat soluble in alkaline solutions and tend to be chemically reduced to metal on separator surfaces, the separator must also inhibit transport and/or chemical reduction of metal ions. Otherwise, a buildup of metal deposits within separator pores can increase the separator resistance in the short term and ultimately lead to shorting failure due to formation of a continuous metal path through the separator. In addition, because of the strong tendency of anodes to form dendrites during charging, the separator must suppress dendritic growth and/or resist dendrite penetration to avoid failure due to formation of a dendritic short between the electrodes. A related issue with anodes is shape change, in which the central part of the electrode tends to thicken during charge-discharge cycling. The causes of shape change are complicated and not well-understood but apparently involve differentials in the current distribution and solution mass transport along the electrode surface. The separator preferably mitigates zinc electrode shape change by exhibiting uniform and stable ionic conductivity and ionic transport properties.
In order to satisfy the numerous and often conflicting separator requirements for zinc-silver oxide batteries, a separator stack comprised of a plurality of separators that perform specific functions is needed. Some of the required functions are resistance to electrochemical oxidation and silver ion transport from the cathode, and resistance to electrochemical reduction and dendrite penetration from the anode.
Traditional separators decompose chemically in alkaline electrolytes, which limits the useful life of the battery. Traditional separators are also subject to chemical oxidation by soluble silver ions and electrochemical oxidation in contact with silver electrodes. Furthermore, some traditional separators exhibit low mechanical strength and poor resistance to penetration by dendrites.
To solve some of the problems caused by traditional separators, new separator materials have been developed.